Ornithologists estimate that up to one billion birds are killed each year by collisions with windows. These birds inadvertently fly into and collide with windows and glass panes in homes and commercial buildings. These collisions usually involve small songbirds; such as finches, that may fall unnoticed to the ground. The collisions are due primarily to the transparent and reflective nature of glass. In the case of transparent glass, birds see an open path, and in the case of reflective glass, birds see images that appear as a mirror showing familiar escape routes and possible safety zones, and thus, birds fly unaware into the window or other glass surface. The speed and force of such an impact can cause instant death of birds. If birds do not die from such impact, they can be left stunned or fatally injured as a result of brain hemorrhages or other internal bleeding.
Bird deaths of this nature have a detrimental environmental impact. Unlike many other environmental problems, there is no apparent offsetting benefit to humans of such deaths. While many bird deaths could be prevented, this problem has received little public attention, and remains largely unrecognized. Klem, D. Jr., “Presentation at North American Ornithological Conference,” St. Louis (Apr. 8, 1998).
Concerned groups, such as the National Audubon Society, have suggested ways to reduce bird deaths resulting from window collisions. Such suggestions include placing decals, objects, or netting on or in front of windows to create an interference pattern to reduce the mirror effect of reflective windows and/or to make transparent windows more visible. However, this can be difficult to do over an entire window or building of windows, and can be unsightly in appearance to homeowners or business owners. Glass may also be tilted, which may provide a temporary, but not a permanent solution. Other proposed solutions include providing glass frits to window or glass panes, so that the window has granules fused thereon to create a frost-like pattern to reduce reflectivity and transparency. However, this is generally cumbersome and can be aesthetically unappealing. Finally, films, such as translucent plastic sheets may be placed on windows to give a more solid appearance. However, this is also not practical on a large scale, can be aesthetically unappealing, and typically will significantly limit visibility to humans.
Other previous attempts to solve the problem of bird collisions are also known. For example, one attempt includes a feather assembly that is made of a length of line including a plurality of brightly colored feathers secured to the line at spaced locations. The line is secured over a glass window in an attempt to prevent bird collisions. Pulse radiation devices have also been used to warn birds of potential hazards such as wind turbine electrical generators. The National Audubon Society's has suggested the use of decals in the shape of spider webs for use with transparent windows. Other similar decals are commercially available from WindowAlert™ of Bend, Oreg. Such decals contain a component that purports to reflect UV sunlight and to act as a neon stoplight for birds. Even if such decals may potentially make a window appear as a solid object without obscuring a human's ability to see through glass, they usually cover only a fraction of a window's surface, leaving the rest of the glass exposed to flying birds. Thus, such decals, to the extent they may be effective, address only part of the problem. Such decals also are not completely transparent or invisible to humans, and do not absorb ultraviolet (UV) light. As a result, they do not appear to be a satisfactory solution to the problems resulting from the transparency of glass over the visual spectrum of birds, the reflective nature of glass and the appearance of bright lights within buildings.
Coatings are known in the art that absorb UV light. They include agents derived from silicones and methoxy-substituted benzophenones as well as organopolysiloxane protective coatings that may contain light absorbing agents. Such coatings, however, are generally directed toward plastic materials to provide UV and scratch resistance.
It is known that many bird species can discriminate wavelengths of light in the near-UV range, which wavelengths are smaller than the threshold wavelengths observable by humans, typically about 400 nm. This is because while humans have three types of photoreceptive cones, birds have an extra cone for quadchromatic color vision. These cones expand their visible light spectrum. Bird eyes, on average, account for about 15% of the mass of the bird's entire head, while human eyes, in contrast, account for less than about 2% of a human head. Furthermore, bird retinas, in contrast to those of humans, do not contain blood vessels, which prevents light scattering such that birds have greater visual acuity activity than humans.
In view of the visual differences between birds and humans and the lack of a satisfactory solution in the art to the problem of bird collisions with windows or other glass surfaces to successfully prevent the significant level of bird deaths, there exists a need in the art for coatings, such as the UV-absorbent coatings described herein which may also be anti-reflective, for windows or other transparent or translucent glass surfaces that are visible to birds, but not to humans. By applying such coatings to windows or other transparent or translucent exterior-facing surfaces, as described herein, such UV-absorbent coatings provide sufficient optical absorption to make them visible to birds, but not to humans, and birds may be deterred from flying into windows.